Metrology method and apparatus, substrate, lithographic system and device manufacturing method

ABSTRACT

A metrology target formed by a lithographic process on a substrate includes a plurality of component gratings. Images of the target are formed using +1 and −1 orders of radiation diffracted by the component gratings. Regions of interest (ROIs) in the detected image are identified corresponding the component gratings. Intensity values within each ROI are processed and compared between images, to obtain a measurement of asymmetry and hence overlay error. Separation zones are formed between the component gratings and design so as to provide dark regions in the image. In an embodiment, the ROIs are selected with their boundaries falling within the image regions corresponding to the separation zones. By this measure, the asymmetry measurement is made more tolerant of variations in the position of the ROI. The dark regions also assist in recognition of the target in the images.

This application incorporates by reference in their entireties U.S.patent application Ser. No. 14/403,010, filed Nov. 21, 2014 which is nowU.S. Pat. No. 9,535,338, Int'l Application No. PCT/EP2013/059061, FiledMay 1, 2013 and U.S. Provisional Application No. 61/652,552, filed May29, 2012.

BACKGROUND

Field of the Present Invention

The present invention relates to methods and apparatus for metrologyusable, for example, in the manufacture of devices by lithographictechniques and to methods of manufacturing devices using lithographictechniques.

Background Art

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,including part of, one, or several dies) on a substrate (e.g., a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

In lithographic processes, it is desirable frequently to makemeasurements of the structures created, e.g., for process control andverification. Various tools for making such measurements are known,including scanning electron microscopes, which are often used to measurecritical dimension (CD), and specialized tools to measure overlay, theaccuracy of alignment of two layers in a device. Recently, various formsof scatterometers have been developed for use in the lithographic field.These devices direct a beam of radiation onto a target and measure oneor more properties of the scattered radiation—e.g., intensity at asingle angle of reflection as a function of wavelength; intensity at oneor more wavelengths as a function of reflected angle; or polarization asa function of reflected angle—to obtain a “spectrum” from which aproperty of interest of the target can be determined. Determination ofthe property of interest may be performed by various techniques: e.g.,reconstruction of the target structure by iterative approaches such asrigorous coupled wave analysis or finite element methods; librarysearches; and principal component analysis.

The targets used by conventional scatterometers are relatively large,e.g., 40 μm by 40 μm, gratings and the measurement beam generates a spotthat is smaller than the grating (i.e., the grating is underfilled).This simplifies mathematical reconstruction of the target as it can beregarded as infinite. However, in order to reduce the size of thetargets, e.g., to 10 μm by 10 μm or less, e.g., so they can bepositioned in amongst product features, rather than in the scribe lane,metrology has been proposed in which the grating is made smaller thanthe measurement spot (i.e., the grating is overfilled). Typically suchtargets are measured using dark field scatterometry in which the zerothorder of diffraction (corresponding to a specular reflection) isblocked, and only higher orders processed. Diffraction-based overlayusing dark-field detection of the diffraction orders enables overlaymeasurements on smaller targets. These targets can be smaller than theillumination spot and may be surrounded by product structures on awafer. Multiple gratings can be measured in one image, using a compositegrating target.

In the known metrology technique, overlay measurement results areobtained by measuring the target twice under certain conditions, whileeither rotating the target or changing the illumination mode or imagingmode to obtain separately the −1^(st) and the +1^(st) diffraction orderintensities. Comparing these intensities for a given grating provides ameasurement of asymmetry in the grating, and asymmetry in an overlaygrating can be used as an indicator of overlay error.

Because of the reduced size of the individual gratings in a compositegrating target, edge effects (fringes) in the dark-field image becomesignificant, and there can be cross-talk between the images of differentgratings within the target. To address this issue, some select only acentral portion of the image of each grating as a ‘region of interest’(ROI). Only pixel values within the ROI are used to calculate asymmetryand overlay. As one considers ever smaller targets, however, the size ofROI that can be defined to be free of edge effects reduces to eversmaller numbers of pixels. Consequently the measurements are inherentlymore noisy, for a given acquisition time. Moreover, any variation inpositioning the ROI becomes a significant source of error in themeasured asymmetry.

SUMMARY

It is desirable to provide a technique for overlay metrology whichmaintains the benefits of using small gratings in composite targetstructures, in which accuracy can be improved over prior publishedtechniques. A particular aim is to avoid the drawbacks associated withselecting smaller ROIs as target size decreases.

The present invention in a first embodiment provides a method ofmeasuring a property of a lithographic process, using a composite targetstructure including a plurality of component structures that have beenformed by the lithographic process on a substrate, the method comprisingthe steps of (a) forming and detecting an image of the composite targetstructure using a predetermined portion of radiation diffracted by thecomponent target structures under predetermined illumination conditions,(b) identifying one or more regions of interest in the detected image,the or each region of interest corresponding to a specific one of thecomponent target structures, and (c) processing pixel values within theregion of interest to obtain a measurement of the property of thecomponent structure. The composite target structure is formed withseparation zones between the component structures so that a variation ofa position of the one or more regions of interest does not significantlyinfluence the obtained measurement of said property.

In some embodiments, the regions in the image corresponding to theseparation zones are used to facilitate recognition of the target in theimage.

In some embodiments, in step (c) the regions of interest are selectedwith their boundaries falling within the image regions corresponding tothe separation zones. The structure in the separation zones can beformed so as to provide image regions that do not vary with the propertybeing measured, so that the measurement is not so sensitive tovariations in the exact positioning of the region of interest.

In some embodiments, the separation zones are formed to as to appeardark in the image, being formed for example with periodic structureshaving spatial frequencies much higher than those in the componentstructures.

The present invention in another embodiment provides an inspectionapparatus for measuring a property of a lithographic process using acomposite target structure including a plurality of component structuresthat have been formed by the lithographic process on a substrate, theapparatus comprising a support for the substrate having the compositetarget structure formed thereon, an optical system for illuminating thecomposite target structure under predetermined illumination conditionsand for forming and detecting an image of the composite target structureusing a predetermined portion of radiation diffracted by the componenttarget structures under the illumination conditions, a processorarranged to identify one or more regions of interest in the detectedimage, the or each region of interest corresponding to a specific one ofthe component target structures and to process pixel values within theregion of interest to obtain a measurement of the property of thecomponent structure. The processor is arranged to identify the regionsof interest such that their boundaries fall within image regionscorresponding to separation zones between the component structureswithin the composite target structure.

The present invention in yet another embodiment provides inspectionapparatus for measuring a property of a lithographic process using acomposite target structure including a plurality of component structuresthat have been formed by the lithographic process on a substrate, theapparatus comprising a support for the substrate having the compositetarget structure formed thereon, an optical system for illuminating thecomposite target structure under predetermined illumination conditionsand for forming and detecting an image of the composite target structureusing a predetermined portion of radiation diffracted by the componenttarget structures under the illumination conditions, a processorarranged to identify one or more regions of interest in the detectedimage, the or each region of interest corresponding to a specific one ofthe component target structures and to process pixel values within theregion of interest to obtain a measurement of the property of thecomponent structure. The processor is arranged to recognize the locationof the composite target and identify the regions of interest at least inpart by recognizing image regions corresponding to separation zonesbetween the component structures within the composite target structure.

The present invention yet further embodiment provides a pair ofpatterning devices for use in forming a substrate according to anyaspect of the present invention as set forth above, the patterningdevices together being adapted for use in forming the composite targetstructure at one or more locations on a substrate. The patterningdevices may be adapted for forming overlaid gratings in two differentlayers on a substrate, or they may be adapted for forming overlaidgratings in a layer by a ‘multiple patterning’ technique.

The present invention yet further embodiment provides a computer programproduct comprising machine-readable instructions for causing a processorto perform the identifying and processing steps (b) and (c) of a methodaccording to the present invention as set forth above.

The present invention yet further embodiment provides a lithographicsystem comprising a lithographic apparatus arranged to transfer asequence of patterns from patterning devices onto a substrate in anoverlying manner; and an inspection apparatus according to the presentinvention as set forth above. The lithographic apparatus is arranged touse the calculated overlay values from the inspection apparatus inapplying the sequence of patterns to further substrates.

A method of manufacturing devices wherein a sequence of device patternsis applied to a series of substrates in an overlying manner using alithographic process, the method including inspecting at least oneperiodic structure formed as part of or beside the device patterns on atleast one of the substrates using an inspection method according to thepresent invention as set forth above, and controlling the lithographicprocess for later substrates in accordance with the calculated overlayerror.

Further features and advantages of the invention, as well as thestructure and operation of various embodiments of the invention, aredescribed in detail below with reference to the accompanying drawings.It is noted that the invention is not limited to the specificembodiments described herein. Such embodiments are presented herein forillustrative purposes only. Additional embodiments will be apparent topersons skilled in the relevant art(s) based on the teachings containedherein.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the relevant art(s) to makeand use the invention.

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe present invention.

FIG. 2 depicts a lithographic cell or cluster according to an embodimentof the present invention.

FIGS. 3A-3D comprise (a) a schematic diagram of a dark fieldscatterometer for use in measuring targets according to embodiments ofthe present invention using a first pair of illumination apertures, (b)a detail of diffraction spectrum of a target grating for a givendirection of illumination (c) a second pair of illumination aperturesproviding further illumination modes in using the scatterometer fordiffraction based overlay measurements and (d) a third pair ofillumination apertures combining the first and second pair of apertures.

FIG. 4 depicts a known form of multiple grating target and an outline ofa measurement spot on a substrate.

FIG. 5 depicts an image of the target of FIG. 4 obtained in thescatterometer of FIG. 3.

FIG. 6 is a flowchart showing the steps of an overlay measurement methodusing the scatterometer of FIG. 3 and adaptable to form an embodiment ofthe present invention.

FIG. 7 illustrates a novel composite grating structure that can be usedin embodiments of the present invention.

FIG. 8 illustrates a dark-field image of the structure of FIG. 7, andillustrates the selection of regions of interest in accordance with thean embodiment of the present invention.

FIGS. 9A-9B illustrate two composite grating structures (a) and (b)having bias schemes that can be used in embodiments of the presentinvention, combining component gratings for two orthogonal directions ofoverlay measurement. and

FIG. 10 shows an array of composite grating structures, for measurementof overlay between several layer pairs in a multilayer device structurebeing manufactured by lithography.

The features and advantages of the present invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings, in which like reference charactersidentify corresponding elements throughout. In the drawings, likereference numbers generally indicate identical, functionally similar,and/or structurally similar elements. The drawing in which an elementfirst appears is indicated by the leftmost digit(s) in the correspondingreference number.

DETAILED DESCRIPTION

This specification discloses one or more embodiments that incorporatethe features of this invention. The disclosed embodiment(s) merelyexemplify the invention. The scope of the invention is not limited tothe disclosed embodiment(s). The invention is defined by the claimsappended hereto.

The embodiment(s) described, and references in the specification to “oneembodiment”, “an embodiment”, “an example embodiment”, etc., indicatethat the embodiment(s) described may include a particular feature,structure, or characteristic, but every embodiment may not necessarilyinclude the particular feature, structure, or characteristic. Moreover,such phrases are not necessarily referring to the same embodiment.Further, when a particular feature, structure, or characteristic isdescribed in connection with an embodiment, it is understood that it iswithin the knowledge of one skilled in the art to effect such feature,structure, or characteristic in connection with other embodimentswhether or not explicitly described.

Embodiments of the invention may be implemented in hardware, firmware,software, or any combination thereof. Embodiments of the invention mayalso be implemented as instructions stored on a machine-readable medium,which may be read and executed by one or more processors. Amachine-readable medium may include any mechanism for storing ortransmitting information in a form readable by a machine (e.g., acomputing device). For example, a machine-readable medium may includeread only memory (ROM); random access memory (RAM); magnetic diskstorage media; optical storage media; flash memory devices; electrical,optical, acoustical or other forms of propagated signals (e.g., carrierwaves, infrared signals, digital signals, etc.), and others. Further,firmware, software, routines, instructions may be described herein asperforming certain actions. However, it should be appreciated that suchdescriptions are merely for convenience and that such actions in factresult from computing devices, processors, controllers, or other devicesexecuting the firmware, software, routines, instructions, etc.

Before describing such embodiments in more detail, however, it isinstructive to present an example environment in which embodiments ofthe present invention may be implemented.

FIG. 1 schematically depicts a lithographic apparatus LA. The apparatusincludes an illumination system (illuminator) IL configured to conditiona radiation beam B (e.g., UV radiation or DUV radiation), a patterningdevice support or support structure (e.g., a mask table) MT constructedto support a patterning device (e.g., a mask) MA and connected to afirst positioner PM configured to accurately position the patterningdevice in accordance with certain parameters; a substrate table (e.g., awafer table) WT constructed to hold a substrate (e.g., a resist coatedwafer) W and connected to a second positioner PW configured toaccurately position the substrate in accordance with certain parameters;and a projection system (e.g., a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g., including one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The patterning device support holds the patterning device in a mannerthat depends on the orientation of the patterning device, the design ofthe lithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The patterning device support can use mechanical, vacuum, electrostaticor other clamping techniques to hold the patterning device. Thepatterning device support may be a frame or a table, for example, whichmay be fixed or movable as required. The patterning device support mayensure that the patterning device is at a desired position, for examplewith respect to the projection system. Any use of the terms “reticle” or“mask” herein may be considered synonymous with the more general term“patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam, which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.,employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g., employing a programmable mirror array of a typeas referred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDincluding, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may include an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL mayinclude various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the patterning device support (e.g., mask tableMT), and is patterned by the patterning device. Having traversed thepatterning device (e.g., mask) MA, the radiation beam B passes throughthe projection system PS, which focuses the beam onto a target portion Cof the substrate W. With the aid of the second positioner PW andposition sensor IF (e.g., an interferometric device, linear encoder, 2-Dencoder or capacitive sensor), the substrate table WT can be movedaccurately, e.g., so as to position different target portions C in thepath of the radiation beam B. Similarly, the first positioner PM andanother position sensor (which is not explicitly depicted in FIG. 1) canbe used to accurately position the patterning device (e.g., mask) MAwith respect to the path of the radiation beam B, e.g., after mechanicalretrieval from a mask library, or during a scan. In general, movement ofthe patterning device support (e.g., mask table) MT may be realized withthe aid of a long-stroke module (coarse positioning) and a short-strokemodule (fine positioning), which form part of the first positioner PM.Similarly, movement of the substrate table WT may be realized using along-stroke module and a short-stroke module, which form part of thesecond positioner PW. In the case of a stepper (as opposed to a scanner)the patterning device support (e.g., mask table) MT may be connected toa short-stroke actuator only, or may be fixed.

Patterning device (e.g., mask) MA and substrate W may be aligned usingmask alignment marks M1, M2 and substrate alignment marks P1, P2.Although the substrate alignment marks as illustrated occupy dedicatedtarget portions, they may be located in spaces between target portions(these are known as scribe-lane alignment marks). Similarly, insituations in which more than one die is provided on the patterningdevice (e.g., mask) MA, the mask alignment marks may be located betweenthe dies. Small alignment markers may also be included within dies, inamongst the device features, in which case it is desirable that themarkers be as small as possible and not require any different imaging orprocess conditions than adjacent features. The alignment system, whichdetects the alignment markers is described further below.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the patterning device support (e.g., mask table) MT andthe substrate table WT are kept essentially stationary, while an entirepattern imparted to the radiation beam is projected onto a targetportion C at one time (i.e., a single static exposure). The substratetable WT is then shifted in the X and/or Y direction so that a differenttarget portion C can be exposed. In step mode, the maximum size of theexposure field limits the size of the target portion C imaged in asingle static exposure.2. In scan mode, the patterning device support (e.g., mask table) MT andthe substrate table WT are scanned synchronously while a patternimparted to the radiation beam is projected onto a target portion C(i.e., a single dynamic exposure). The velocity and direction of thesubstrate table WT relative to the patterning device support (e.g., masktable) MT may be determined by the (de-)magnification and image reversalcharacteristics of the projection system PS. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.3. In another mode, the patterning device support (e.g., mask table) MTis kept essentially stationary holding a programmable patterning device,and the substrate table WT is moved or scanned while a pattern impartedto the radiation beam is projected onto a target portion C. In thismode, generally a pulsed radiation source is employed and theprogrammable patterning device is updated as required after eachmovement of the substrate table WT or in between successive radiationpulses during a scan. This mode of operation can be readily applied tomaskless lithography that utilizes programmable patterning device, suchas a programmable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

Lithographic apparatus LA is of a so-called dual stage type which hastwo substrate tables WTa, WTb and two stations—an exposure station and ameasurement station—between which the substrate tables can be exchanged.While one substrate on one substrate table is being exposed at theexposure station, another substrate can be loaded onto the othersubstrate table at the measurement station and various preparatory stepscarried out. The preparatory steps may include mapping the surfacecontrol of the substrate using a level sensor LS and measuring theposition of alignment markers on the substrate using an alignment sensorAS. This enables a substantial increase in the throughput of theapparatus. If the position sensor IF is not capable of measuring theposition of the substrate table while it is at the measurement stationas well as at the exposure station, a second position sensor may beprovided to enable the positions of the substrate table to be tracked atboth stations.

As shown in FIG. 2, the lithographic apparatus LA forms part of alithographic cell LC, also sometimes referred to a lithocell or cluster,which also includes apparatus to perform pre- and post-exposureprocesses on a substrate. Conventionally these include spin coaters SCto deposit resist layers, developers DE to develop exposed resist, chillplates CH and bake plates BK. A substrate handler, or robot, RO picks upsubstrates from input/output ports I/O1, I/O2, moves them between thedifferent process apparatus and delivers then to the loading bay LB ofthe lithographic apparatus. These devices, which are often collectivelyreferred to as the track, are under the control of a track control unitTCU which is itself controlled by the supervisory control system SCS,which also controls the lithographic apparatus via lithography controlunit LACU. Thus, the different apparatus can be operated to maximizethroughput and processing efficiency.

Examples of dark field metrology can be found in international patentapplications WO 2009/078708 and WO 2009/106279 which documents arehereby incorporated by reference in their entirety. Further developmentsof the technique have been described in published patent publicationsUS20110027704A, US20110043791A, US20120044470 and US2012/0123581, and inthe following U.S. patent application Ser. Nos. 14/825,751 and14/934,734. The contents of all these applications are also incorporatedherein by reference.

A dark field metrology apparatus suitable for use in embodiments of thepresent invention is shown in FIG. 3(a). A grating target T anddiffracted rays are illustrated in more detail in FIG. 3(b). The darkfield metrology apparatus may be a stand-alone device or incorporated ineither the lithographic apparatus LA, e.g., at the measurement station,or the lithographic cell LC. An optical axis, which has several branchesthroughout the apparatus, is represented by a dotted line O. In thisapparatus, light emitted by source 11 (e.g., a xenon lamp) is directedonto substrate W via a beam splitter 15 by an optical system comprisinglenses 12, 14 and objective lens 16. These lenses are arranged in adouble sequence of a 4F arrangement. A different lens arrangement can beused, provided that it still provides a substrate image onto a detector,and simultaneously allows for access of an intermediate pupil-plane forspatial-frequency filtering. Therefore, the angular range at which theradiation is incident on the substrate can be selected by defining aspatial intensity distribution in a plane that presents the spatialspectrum of the substrate plane, here referred to as a (conjugate) pupilplane. In particular, this can be done by inserting an aperture plate 13of suitable form between lenses 12 and 14, in a plane which is aback-projected image of the objective lens pupil plane. In the exampleillustrated, aperture plate 13 has different forms, labeled 13N and 13S,allowing different illumination modes to be selected. The illuminationsystem in the present examples forms an off-axis illumination mode. Inthe first illumination mode, aperture plate 13N provides off-axis from adirection designated, for the sake of description only, as ‘north’. In asecond illumination mode, aperture plate 13S is used to provide similarillumination, but from an opposite direction, labeled ‘south’. Othermodes of illumination are possible by using different apertures. Therest of the pupil plane is desirably dark as any unnecessary lightoutside the desired illumination mode will interfere with the desiredmeasurement signals.

As shown in FIG. 3(b), grating target T is placed with substrate Wnormal to the optical axis O of objective lens 16. A ray of illuminationI impinging on target T from an angle off the axis O gives rise to azeroth order ray (solid line 0) and two first order rays (dot-chain line+1 and double dot-chain line −1). It should be remembered that with anoverfilled small target grating, these rays are just one of manyparallel rays covering the area of the substrate including metrologytarget T and other features. Where a composite grating target isprovided, each individual grating within the target will give rise toits own diffraction spectrum. Since the aperture in plate 13 has afinite width (necessary to admit a useful quantity of light, theincident rays I will in fact occupy a range of angles, and thediffracted rays 0 and +1/−1 will be spread out somewhat. According tothe point spread function of a small target, each order +1 and −1 willbe further spread over a range of angles, not a single ideal ray asshown. Note that the grating pitches and illumination angles can bedesigned or adjusted so that the first order rays entering the objectivelens are closely aligned with the central optical axis. The raysillustrated in FIGS. 3(a) and 3(b) are shown somewhat off axis, purelyto enable them to be more easily distinguished in the diagram.

At least the 0 and +1 orders diffracted by the target on substrate W arecollected by objective lens 16 and directed back through beam splitter15. Returning to FIG. 3(a), both the first and second illumination modesare illustrated, by designating diametrically opposite apertures labeledas north (N) and south (S). When the incident ray I is from the northside of the optical axis, that is when the first illumination mode isapplied using aperture plate 13N, the +1 diffracted rays, which arelabeled +1(N), enter the objective lens 16. In contrast, when the secondillumination mode is applied using aperture plate 13S the −1 diffractedrays (labeled −1(S)) are the ones which enter the lens 16.

A second beam splitter 17 divides the diffracted beams into twomeasurement branches. In a first measurement branch, optical system 18forms a diffraction spectrum (pupil plane image) of the target on firstsensor 19 (e.g., a CCD or CMOS sensor) using the zeroth and first orderdiffractive beams. Each diffraction order hits a different point on thesensor, so that image processing can compare and contrast orders. Thepupil plane image captured by sensor 19 can be used for focusing themetrology apparatus and/or normalizing intensity measurements of thefirst order beam. The pupil plane image can also be used for manymeasurement purposes such as reconstruction, which are not the subjectof the present disclosure.

In the second measurement branch, optical system 20, 22 forms an imageof the target on the substrate W on sensor 23 (e.g., a CCD or CMOSsensor). In the second measurement branch, an aperture stop 21 isprovided in a plane that is conjugate to the pupil-plane. Aperture stop21 functions to block the zeroth order diffracted beam so that the imageof the target formed on sensor 23 is formed only from the −1 or +1 firstorder beam. The images captured by sensors 19 and 23 are output to imageprocessor and controller PU, the function of which will depend on theparticular type of measurements being performed. Note that the term‘image’ is used here in a broad sense. An image of the grating lines assuch will not be formed, if only one of the −1 and +1 orders is present.

The particular forms of aperture plate 13 and field stop 21 shown inFIG. 3 are purely examples. In another embodiment of the presentinvention, on-axis illumination of the targets is used, and an aperturestop with an off-axis aperture is used to pass substantially only onefirst order of diffracted light to the sensor. (The apertures shown at13 and 21 are effectively swapped in that case.) In yet otherembodiments, 2nd, 3rd and higher order beams (not shown in FIG. 3) canbe used in measurements, instead of or in addition to the first orderbeams.

In order to make the illumination adaptable to these different types ofmeasurement, the aperture plate 13 may comprise a number of aperturepatterns formed around a disc, which rotates to bring a desired patterninto place. Alternatively or in addition, a set of plates 13 could beprovided and swapped, to achieve the same effect. A programmableillumination device such as a deformable mirror array or transmissivespatial light modulator can be used also. Moving mirrors or prisms canbe used as another way to adjust the illumination mode.

As just explained in relation to aperture plate 13, the selection ofdiffraction orders for imaging can alternatively be achieved by alteringthe pupil-stop 21, or by substituting a pupil-stop having a differentpattern, or by replacing the fixed field stop with a programmablespatial light modulator. In that case the illumination side of themeasurement optical system can remain constant, while it is the imagingside that has first and second modes. In the present disclosure,therefore, there are effectively three types of measurement method, eachwith its own advantages and disadvantages. In one method, theillumination mode is changed to measure the different orders. In anothermethod, the imaging mode is changed. In a third method, the illuminationand imaging modes remain unchanged, but the target is rotated through180 degrees. In each case the desired effect is the same, namely toselect first and second portions of the non-zero order diffractedradiation which are symmetrically opposite one another in thediffraction spectrum of the target. In principle, the desired selectionof orders could be obtained by a combination of changing theillumination modes and the imaging modes simultaneously, but that islikely to bring disadvantages for no advantage, so it will not bediscussed further.

While the optical system used for imaging in the present examples has awide entrance pupil which is restricted by the field stop 21, in otherembodiments or applications the entrance pupil size of the imagingsystem itself may be small enough to restrict to the desired order, andthus serve also as the field stop. Different aperture plates are shownin FIGS. 3(c) and (d) which can be used as described further below.

Typically, a target grating will be aligned with its grating linesrunning either north-south or east-west. That is to say, a grating willbe aligned in the X direction or the Y direction of the substrate W.Note that aperture plate 13N or 13S can only be used to measure gratingsoriented in one direction (X or Y depending on the set-up). Formeasurement of an orthogonal grating, rotation of the target through 90°and 270° might be implemented. More conveniently, however, illuminationfrom east or west is provided in the illumination optics, using theaperture plate 13E or 13W, shown in FIG. 3(c). The aperture plates 13Nto 13W can be separately formed and interchanged, or they may be asingle aperture plate which can be rotated by 90, 180 or 270 degrees. Asmentioned already, the off-axis apertures illustrated in FIG. 3(c) couldbe provided in field stop 21 instead of in illumination aperture plate13. In that case, the illumination would be on axis.

FIG. 3(d) shows a third pair of aperture plates that can be used tocombine the illumination modes of the first and second pairs. Apertureplate 13NW has apertures at north and east, while aperture plate 13SEhas apertures at south and west. Provided that cross-talk between thesedifferent diffraction signals is not too great, measurements of both Xand Y gratings can be performed without changing the illumination mode.

Overlay Measurement Using Small Targets

FIG. 4 depicts a composite grating target formed on a substrateaccording to known practice. The composite target comprises fourindividual gratings 32 to 35 positioned closely together so that theywill all be within a measurement spot 31 formed by the illumination beamof the metrology apparatus. The four targets thus are all simultaneouslyilluminated and simultaneously imaged on sensors 19 and 23. In anexample dedicated to overlay measurement, gratings 32 to 35 arethemselves composite gratings formed by overlying gratings that arepatterned in different layers of the semiconductor device formed onsubstrate W. Gratings 32 to 35 may have differently biased overlayoffsets in order to facilitate measurement of overlay between the layersin which the different parts of the composite gratings are formed.Gratings 32 to 35 may also differ in their orientation, as shown, so asto diffract incoming radiation in X and Y directions. In one example,gratings 32 and 34 are X-direction gratings with biases of the +d, −d,respectively. This means that grating 32 has its overlying componentsarranged so that if they were both printed exactly at their nominallocations one of the components would be offset relative to the other bya distance d. Grating 34 has its components arranged so that ifperfectly printed there would be an offset of d but in the oppositedirection to the first grating and so on. Gratings 33 and 35 areY-direction gratings with offsets +d and −d respectively. While fourgratings are illustrated, another embodiment might require a largermatrix to obtain the desired accuracy. For example, a 3×3 array of ninecomposite gratings may have biases −4d, −3d, −2d, −d, 0, +d, +2d, +3d,+4d. Separate images of these gratings can be identified in the imagecaptured by sensor 23.

FIG. 5 shows an example of an image that may be formed on and detectedby the sensor 23, using the target of FIG. 4 in the apparatus of FIG. 3,using the aperture plates 13NW or 13SE from FIG. 3(d). While the pupilplane image sensor 19 cannot resolve the different individual gratings32 to 35, the image sensor 23 can do so. The cross-hatched rectangle 40represents the field of the image on the sensor, within which theilluminated spot 31 on the substrate is imaged into a correspondingcircular area 41. Ideally the field is dark. Within this dark fieldimage, rectangular areas 42-45 represent the images of the individualgratings 32 to 35. If the gratings are located in product areas, productfeatures may also be visible in the periphery of this image field. Whileonly a single composite grating target is shown in the dark field imageof FIG. 5, in practice a semiconductor device or other product made bylithography may have many layers, and overlay measurements are desiredto be made between different pairs of layers. For each overlaymeasurement between pair of layers, one or more composite gratingtargets are required, and therefore other composite grating targets maybe present, within the image field. Image processor and controller PUprocesses these images using pattern recognition to identify theseparate images 42 to 45 of gratings 32 to 35. In this way, the imagesdo not have to be aligned very precisely at a specific location withinthe sensor frame, which greatly improves throughput of the measuringapparatus as a whole. However the need for accurate alignment remains ifthe imaging process is subject to non-uniformities across the imagefield. In one embodiment of the present invention, four positions P1 toP4 are identified and the gratings are aligned as much as possible withthese known positions.

Once the separate images of the gratings have been identified, theintensities of those individual images can be measured, e.g., byaveraging or summing selected pixel intensity values within theidentified areas. Intensities and/or other properties of the images canbe compared with one another. These results can be combined to measuredifferent parameters of the lithographic process. Overlay performance isan important example of such a parameter.

FIG. 6 illustrates how, using for example the method described inapplication US 2011/027704, overlay error between the two layerscontaining the component gratings 32 to 35 is measured through asymmetryof the gratings, as revealed by comparing their intensities in the +1order and −1 order dark field images. At step S1, the substrate, forexample a semiconductor wafer, is processed through the lithographiccell of FIG. 2 one or more times, to create a structure including theoverlay targets 32-35. At S2, using the metrology apparatus of FIG. 3,an image of the gratings 32 to 35 is obtained using only one of thefirst order diffracted beams (say −1). Then, whether by changing theillumination mode, or changing the imaging mode, or by rotatingsubstrate W by 180° in the field of view of the metrology apparatus, asecond image of the gratings using the other first order diffracted beam(+1) can be obtained (step S3). Consequently the +1 diffracted radiationis captured in the second image.

Note that, by including only half of the first order diffractedradiation in each image, the ‘images’ referred to here are notconventional dark field microscopy images. Each grating will berepresented simply by an area of a certain intensity level. Theindividual grating lines will not be resolved, because only one of the+1 and −1 order diffracted radiation is present. In step S4, a region ofinterest (ROI) is carefully identified within the image of eachcomponent grating, from which intensity levels will be measured. This isdone because, particularly around the edges of the individual gratingimages, intensity values can be highly dependent on process variablessuch as resist thickness, composition, line shape, as well as edgeeffects generally.

The present application discloses a new approach to target design andROI selection, as will be described in more detail below, with referenceto FIG. 7 onwards.

Having identified the ROI for each individual grating and measured itsintensity, the asymmetry of the grating structure, and hence overlayerror, can then be determined. This is done by the image processor andcontroller PU in step S5 comparing the intensity values obtained for +1and −1 orders for each grating 32-35 to identify any difference in theirintensity, and (S6) from knowledge of the overlay biases of the gratingsto determine overlay error in the vicinity of the target T.

In the prior applications, mentioned above, various techniques aredisclosed for improving the quality of overlay measurements using thebasic method mentioned above. For example, the intensity differencesbetween images may be attributable to differences in the optical pathsused for the different measurements, and not purely asymmetry in thetarget. The illumination source 11 may be such that the intensity and/orphase of illumination spot 31 is not uniform. Corrections can thedetermined and applied to minimize such errors, by reference for exampleto the position of the target image in the image field of sensor 23. Theindividual component gratings may be elongated in their direction ofperiodicity, so as to maximize the useful diffraction signals within agiven target area. These techniques are explained in the priorapplications, and will not be explained here in further detail. They maybe used in combination with the techniques newly disclosed in thepresent application, which will now be described.

In another patent application (Ser. No. 14/934,734), features in andaround edge portions of the individual gratings are modified so as toreduce the intensity and extent of edge effects. These modifications maywork in a similar way to optical proximity correction (OPC) featuresused to enhance the printing of fine features in a lithographic process.In another application (Ser. No. 14/825,751), it is proposed to usethree or more component gratings to measure overlay by the method ofFIG. 6. By measuring asymmetries for gratings with at least threedifferent biases, the calculations in step S6 can be modified so as tocorrect for feature asymmetry in the target gratings, such as is causedby bottom grating asymmetry in a practical lithographic process. Thesetechniques similarly are explained in the prior applications, and willnot be explained here in further detail. They may be used in combinationwith the techniques newly disclosed in the present application, whichwill now be described.

Edge Effects and ROI Selection

While FIG. 5 shows an idealized image with four squares 42-45 of uniformintensity, in practice, however, the image of each grating on the camerais not perfect. Due to the nature of the dark-field imaging, the edgesof the target light up more brightly than the center part. This makes itdifficult to measure “the” intensity of the target. Furthermore, lightcontribution from the neighboring grating, or the surrounding, needs tobe avoided. In order to solve this problem currently a region ofinterest (ROI) is defined, which excludes the edges, and only selectslight from the central part of each of the four component gratings.However, this means that effectively the signal is collected from asmaller area than the full size of the gratings. For example, where theindividual gratings are 5×5 μm square, the ROI may correspond to only a3×3 μm square area in the middle of the grating image. This decrease insignal either needs to be compensated with a longer acquisition time orresults in a larger measurement uncertainty. Furthermore, correctplacement of the ROI on the grating is extremely critical. A small shiftwill result in part of the edge light being included, which will lead torelatively large changes in the detected intensity and thus deterioratemeasurement precision and accuracy further.

In this disclosure we propose a new target design. As a first benefit,the new target design can permit easier recognition of the target in thedetected image. As a second benefit, when the new target is used incombination with a new ROI selection method the tolerance for the exactplacement of the ROI can be improved. Potentially also the signal areacan be increased, even as the size of the component gratings is beingreduced.

A first novel feature of the proposed technique is to provide aseparation area between the component gratings that make up thecomposite grating target. This separation can be made sufficientlylarge, that the intensity in between the images of the component targetsis not significantly influenced by the component gratings, in spite ofthe presence of diffraction and edge fringes. The separation zones maybe formed for example so that the intensity in those parts of the darkfield image drops to substantially lower values, and appears dark. Thisfacilitates the recognition of the composite target image and thereforethe selection of the ROI. Example target designs will be presentedbelow.

Additionally, the separation zones can be formed so that, should partsof them be included within the ROI, they provide no signal with respectto the property being measured. The composite target structure may beformed with separation zones between the component structures so that avariation of a position of one or more regions of interest does notsignificantly influence the measurement of the property. A second novelfeature in some embodiments is in the way the resulting data isprocessed. In particular, there is a change in the way the region ofinterest (ROI) is selected in step S4 of the method steps shown in FIG.6. In conjunction with use of the new target design, the ROI in thenovel technique is chosen to be larger than the grating image, anddeliberately includes all the fringes. The relatively large separationbetween the gratings allows the border of the ROI to extend throughparts of the image corresponding to the separation zones. Because theseparation zones are designed so as to provide no signal relevant to themeasurement of interest, this reduces the sensitivity to the exactplacement of the ROI. An easy way to make the separation zones provideno signal is to make them appear much darker than the component targets,as already mentioned. In principle, however, they do not need to appeardark, in order to have no influence on the measurement of the propertyof interest. For example, if pixel intensity values within theseparation zone image regions are generally constant and insensitive tooverlay variations, then the overlay measurement result will still betolerant of variations in the exact placement of the ROI.

FIG. 7 shows a composite grating target of new design. As in the knowntarget of FIG. 4, there are four rectangular (optionally square)component gratings, 32′, 33′, 34′ and 35′. The form and layout of thesegratings is similar to that of the gratings 32-35 in FIG. 4, but theyare separated from one another and from their environment by separationzones 80 and 82. These separation zones are formed so as to form clear,dark regions in the dark field images detected by sensor 23. In orderfor the target to print and to process correctly, preferably theseparation zones 80 and 82 are not completely blank, but filled with a‘dummy’ structure. This dummy structure could be for example a gratingwith a much smaller pitch than the target gratings, but a comparabledensity. In this way the etch load (ratio of exposed to unexposedresist) of the dummy gratings is similar to that of the target gratings.Because of the much smaller pitch, the angle of +1 and −1 orderdiffraction from the dummy features is much greater than for the targetgratings, so that light diffracted by the dummy structure will not passthe dark-field pupil stop and thus will not perturb the metrologymeasurement.

Assuming for simplicity that the composite grating target is square,representative dimensions a, b, c and e are marked to one side of thestructure. Within a square of side a, component gratings with side b areseparated by a separation zone 80 of width c. Separation zones 82 ofwidth e surround the four component gratings. Assuming that thecomposite target will be arrayed with similar composite targets,dimension e can be smaller than dimension c, for example half the size.Rather than make the composite grating target larger to accommodate theseparation zones, it is proposed that the gratings are reduced in size,to create the separation zones with no overall increase in the dimensiona of the composite target.

As an example of possible dimensions, in one embodiment four gratings32′-35′ with dimension b=4 μm are placed in a composite target area witha=10 μm. The separation c between the gratings is 1 μm, and theseparation e between a grating and the surroundings is 0.5 μm. If suchtargets are placed side by side (as illustrated in FIG. 11) this resultsin a 1 μm separation between gratings within different compositetargets. The pitches of the gratings may be for example in the range350-1050 μm. For a simple grating printed in overlaid layers, the linewidth within the grating is typically 50% of the pitch, though this isnot essential. For example the pitch may be 500 nm (0.5 μm), with linewidths for example 250 nm. As another example, the pitch may be 600 nm(0.6 μm). The optimum pitch will be a function of the apertures andpupil dimension, and the wavelengths of radiation to be used for themeasurement. The skilled reader will know that the ideal line width candeviate from 50%, when optimized for linearity, signal strength and thelike. Other forms of overlaid gratings are possible, not just within twolayers. For example, two lithography steps may be applied to form agrating a single resist layer or product layer. In particular, in adouble patterning process, multiple lithography and/or processing stepsare applied to provide interleaved features of smaller pitch than can beformed using a single lithography step. In each reticle, the line widthof the lines may then be much smaller than 50% of the overall pitch. Theoverlay error and associated asymmetry then arise within a single resistlayer or product layer, rather than between two layers. The measurementtechniques described herein can be applied equally in that case. Theterm ‘overlaid’ gratings and ‘overlay’ are to be understood as coveringgratings formed in a single resist layer or product layer by a multiplepatterning (e.g., double patterning) process.

As is well known in the art, these metrology grating features may havedimensions be much larger than critical dimensions of product featuresin the device being manufactured. The wavelengths of light used for themeasurements may be much longer than that used for the exposure in thelithographic apparatus. Within the separation zones, the dummystructures may have dimensions similar to the product features.

If needed, the separations c and e between the targets and thesurroundings may be increased. To limit the total required area for thecomposite grating, the gratings may be reduced to for example b=3 μm.This will reduce the amount of signal from these gratings somewhat, butit should be recalled that the ROI is only 3×3 μm for a 5×5 μm grating,in the known techniques.

The smaller number of lines within the gratings makes the relativecontribution of the edge-lines to the total grating area larger.Therefore, matching between the optical proximity correction-like(OPC-like) features applied to the grating, and with the dummystructures, needs to be taken into account, in order to optimize forcorrect first and last line printing. (In practice, the detailed form ofthe grating lines and dummy structures will be optimized at everytarget, according to the exposure conditions of that particular productlayer.)

FIG. 8 illustrates schematically a dark field image corresponding toFIG. 5, for the target of FIG. 7 and using illumination from twoorthogonal directions. A part of the image field 40′ corresponding tothe target of interest is illustrated within a dashed boundaryrectangle. Areas 42′ to 45′ of the image field correspond to theindividual grating targets. Compared with the idealized, homogeneousareas illustrated in FIG. 5, FIG. 8 shows more realistically the widevariations in intensity caused by edge effects. In the area 45′, forexample, we see a relatively uniform central area 45′a fringed by muchbrighter areas 45′b. Surrounding these elements of the grating image 45′are darker regions 90 and 92, corresponding to the separation zones 80and 82 respectively in the grating target. We also see that, due todiffraction, the influence of each component grating extends outside theimage area strictly corresponding to the component grating. However, theseparation zones in the target are wide enough that the dark imageregions 90 and 92 contain substantially no contribution from thecomponent gratings. The dark regions 90 and 92 are naturally a littlenarrower than would be implied strictly by the dimensions of theseparation zones. The necessary width of separation zone can be reducedsomewhat if the edge effects and diffraction are well-controlled bymodifications of the grating structure.

According to the new principles disclosed herein, a region of interest(ROI) bounded by the white dotted square 94 is selected that includesthe entire grating image 45′ and parts of the surrounding dark regions90, 92. The boundaries of the ROI fall in region that are much darkerthan any of the image features of interest. Therefore any slight errorsin placement of the ROI boundaries have very little impact on themeasured intensity within the ROI as a whole. In the known technique, bycontrast, a much smaller ROI such as that indicated by the black dashedrectangle 96 would have been chosen, trying to keep within thehomogeneous area 45′a only. Because the boundaries of the rectangle 96fall within relatively bright and inhomogeneous parts of the overallgrating image 45′, the measured intensity using the known ROI selectiontechnique is highly sensitive to variations in the exact positioning ofthe rectangle or other boundary. Accuracy and repeatability of themeasurements can become degraded, especially as one tries to shrink thetargets into smaller spaces.

How dark the separation zones can be made to appear is a matter ofdesign choice and compromise between constraints. Forpattern-recognition purposes, it may be very convenient to have theseregions as dark as possible. This will occur if the pitches of dummystructures in the separation zones are small enough such that thegenerated diffraction orders of these zone-structures are all filteredout, not transmitted to the camera. They will be blocked by field stop21, and may even be blocked by objective 16. From the point of view ofthe measurement signal. It is important that the asymmetry in theintensity (difference between +1 and −1 order intensities) goes to zero,not necessarily the intensity itself. However, this “background”intensity should not have variations like a gradient or a left-rightasymmetry such that ROI position errors again contribute to asymmetry inthe signal. The safest way is therefore indeed to have intensities thatare nominally zero in the separation regions.

A criterion for design in this regard could be that the intensityvariation in the zones should be below 0.1% of the asymmetry signal. Ifthe asymmetry signal is, for example, 10% of the intensity within thegrating, then the variation in intensity throughout the separation zonesshould be below 10⁻⁴ of the intensity in the grating image area.Conversely, this is not a hard requirement, and without meeting thiscriterion the measurement may still be useable. One can say for examplethat the intensity variation in the image of the separation zones shouldbe below 10%, optionally below 1%, or even below 0.1%.

Referring back to the numerical examples presented above, the dimensionb of the component gratings may be for example 3 or 4 μm, while the ROIdimension corresponds to 4 or 5 μm. Thus in each direction theseparation zones may occupy more than 5%, more than 10% or even morethan 15% or 20% of the composite target dimension. Where the compositetarget is positioned next to another composite target on the substrate,the separation zones at the edges of these composite targets may beconsidered to overlap one another, for the purposes of measuring thesepercentage criteria. As mentioned above, measures can also be taken indesigning the target to reduce the strength of the edge effects, andthese measures are by no means excluded from use with the presenttechnique.

FIGS. 9 (a) and (b) show alternative forms of composite grating target,having more than two gratings per orientation. Such targets can be usedto implement overlay measurement with BGA correction, using theprinciples discussed in prior patent application U.S. 61/616,398,mentioned above. In FIG. 9(a) there are three X-direction gratings andthree Y-direction gratings, within a rectangular area of dimension a by3a/2. In FIG. 9 (b) there are four component gratings in each direction(total eight gratings), within a rectangular area of dimension a by 2a.As with the example of FIG. 7, separation zones are provided between andaround the component gratings within the composite target area. Theoverlay bias schemes are indicated in FIG. 9. Target (a) has threegratings per direction with the biases +d, 0, −d. Target (b) has fourgratings with different permutations of bias value d and a sub-biasvalue Δd. The distinction between bias and sub-bias values is a matterof convenient notation. The biases for the four gratings in target (b)can be rewritten as ±d₁, ±d₂, where ±d₁=±(d−Δd), and ±d₂=±(d+Δd).

In the examples of FIG. 9, the X and Y gratings with each bias value areside-by-side, though that is not essential. The X- and Y-directiongratings are interspersed with one another in an alternating pattern, sothat different X gratings are diagonally spaced, not side-by-side withone another, and Y gratings are diagonally spaced, not side-by-side withone another. This arrangement may help to reduce cross-talk betweendiffraction signals of the different biased gratings. The wholearrangement thus allows a compact target design, without goodperformance.

The increased spacing between component gratings, and the resultant darkregions between grating images in the dark field image of the target,make it easier to recognize the location of the grating images andidentify the ROIs. In the examples described above and illustrated inFIGS. 7 to 9, all the gratings are square, and the intermediate spaceforms a normal cross. In another embodiment these gratings may be placedslightly off the square grid, or may be rectangular in shape in order tobreak the symmetry of the target. This may improve the accuracy &robustness of the pattern recognition algorithm that is used to find thetargets in the images even further. Composite grating structures withelongate gratings are described for example in published patentapplication US20120044470.

FIG. 10 illustrates a number of composite grating targets laid out in ametrology area 900 on the substrate of a semiconductor device beingmanufactured using the lithographic and metrology apparatus of FIGS. 1and 2. The metrology area may comprise a square array of N×N compositetargets, of which one is illustrated within dashed square 902. Whereeach composite target occupies an area 10×10 μm, as in the exampleabove, an array of 6×6 targets can be arranged in a metrology area ofsize 60×60 μm. This metrology area may be within a scribe lane betweenproduct areas C on substrate W, and it may be within a product area.Assuming each composite target has separation zones with dimensionssimilar to those in FIG. 7, an outer separation zone 904 of width e′ isadded, to ensure a desired minimum separation between the targetgratings and surrounding product features. Note that the separationzones in FIG. 10 are left white just for clarity. In practice they willbe filled with dummy structures in the same manner as the previousembodiments.

The skilled reader will appreciate that all of the gratings shown inFIG. 10 would not be present in the same layer pairs of thesemiconductor or other product. Rather, one of the composite targetswill exist in the patterns of two layers whose relative alignment(overlay) is to be measured. For those two layers a respective pair ofpatterning devices, such as reticles, will be formed with grating linesand associated features in the appropriate positions, and with thepositions of those grating lines offset in accordance with the desiredbias scheme. The positions for other composite targets in the area 900will be left blank for use in other layers. Dummy features may beincluded in such ‘blank’ areas, to avoid crosstalk between layers. Asmentioned already, features can be designed into the targets tofacilitate pattern recognition. In the illustration of FIG. 10, X and Ydirection gratings are elongated slightly in X and Y directions. Thisbreaks somewhat the symmetry of the overall pattern, making it lesslikely that the dark field image areas will be mis-recognized in step S4of the measurement method. Instead or in addition to breaking thesymmetry in this way, special markers could be placed within theseparation zones, to aid recognition.

CONCLUSION

The technique disclosed herein enable the design and use of smallmetrology targets to achieve great accuracy and repeatability of overlaymeasurements. Particular benefits that may be realized in a particularimplementation include: reduced ROI positioning sensitivity, thereforebetter repeatability of measurements; reduced focus-sensitivity due toincreased space between the gratings, leading to more accurate overlayvalues and/or greater throughput; reduced grating-to-grating cross-talkdue to larger space between the gratings leading to more accurateoverlay values; pattern-recognition improved due to better recognizabletarget imaged with dark separation zones; larger ROI meaning largereffective area for the measurement of pixel intensities, yielding betterreproducibility.

The technique is compatible with other techniques in small targetdiffraction based overlay measurement, that have been described in therecent published and unpublished patent applications mentioned above.For example, using composite targets with three or more different biasvalues per direction, calculations can yield a BGA-corrected overlaymeasurement without the need for modeling the top & bottom gratings orany intervening layers such as BARC (antireflective coating).

While the target structures described above are metrology targetsspecifically designed and formed for the purposes of measurement, inother embodiments, properties may be measured on targets which arefunctional parts of devices formed on the substrate. Many devices haveregular, grating-like structures. The terms ‘target grating’ and ‘targetstructure’ as used herein do not require that the structure has beenprovided specifically for the measurement being performed.

In association with the physical grating structures of the targets asrealized on substrates and patterning devices, an embodiment may includea computer program containing one or more sequences of machine-readableinstructions describing a methods of producing targets on a substrate,measuring targets on a substrate and/or analyzing measurements to obtaininformation about a lithographic process. This computer program may beexecuted for example within unit PU in the apparatus of FIG. 3 and/orthe control unit LACU of FIG. 2. There may also be provided a datastorage medium (e.g., semiconductor memory, magnetic or optical disk)having such a computer program stored therein. Where an existingmetrology apparatus, for example of the type shown in FIG. 3, is alreadyin production and/or in use, the present invention can be implemented bythe provision of updated computer program products for causing aprocessor to perform the modified step S4 and so calculate overlay errorwith reduced sensitivity to ROI positioning errors. The program mayoptionally be arranged to control the optical system, substrate supportand the like to perform the steps S2-S5 for measurement of asymmetry ona suitable plurality of target structures.

Further embodiments according to the invention are provided in belownumbered clauses:

1. A method of measuring a property of a lithographic process, using acomposite target structure including a plurality of component structuresthat have been formed by the lithographic process on a substrate, themethod comprising the steps of:

(a) forming and detecting an image of the composite target structureusing a predetermined portion of radiation diffracted by the componenttarget structures under predetermined illumination conditions;

(b) identifying one or more regions of interest in the detected image,the or each region of interest corresponding to a specific one of thecomponent target structures; and

(c) processing pixel values within the region of interest to obtain ameasurement of the property of the component structure,

wherein the composite target structure is formed with separation zonesbetween the component structures so that a variation of a position ofthe one or more regions of interest does not significantly influence theobtained measurement of said property.

2. A method of clause 1 wherein in step (c) the regions of interest areselected with their boundaries falling within the image regionscorresponding to the separation zones.

3. A method of clause 1 or 2 wherein the component structures compriseoverlaid gratings, and wherein different component structures within thecomposite target are formed with different overlay bias values.

4. A method of clause 3 wherein the component structures compriseoverlaid gratings, and wherein different component structures within thecomposite target are formed with different orientations to measureoverlay in different directions.

5. A method of clause 1, 2, 3 or 4 wherein two or more images of thecomposite target structure are detected using different portions of thediffracted radiation, and wherein step (e) comprises comparing the pixelvalues from corresponding regions of interest identified in the imagesto obtain a measurement of asymmetry of the one or more componentstructures.6. A method of any preceding clause wherein in the steps (b) and (c)regions of interest corresponding to at least two component structuresare identified in the same detected image and their pixel values areprocessed separately.7. A method of any preceding clause wherein the separation zones occupymore than 5%, optionally more than 10% or more than 15% of the compositestructure in a given direction.8. A method of any preceding clause wherein the separation zones in thecomposite target structure contain filling structures having an averagedensity similar to that of the component structures but with higherspatial frequencies, whereby radiation diffracted by the fillingstructures falls outside the portion of radiation used in the formationof the detected image.9. An inspection apparatus for measuring a property of a lithographicprocess using a composite target structure including a plurality ofcomponent structures that have been formed by the lithographic processon a substrate, the apparatus comprising:

a support for the substrate having the composite target structure formedthereon;

an optical system for illuminating the composite target structure underpredetermined illumination conditions and for forming and detecting animage of the composite target structure using a predetermined portion ofradiation diffracted by the component target structures under theillumination conditions;

a processor arranged to identify one or more regions of interest in thedetected image, the or each region of interest corresponding to aspecific one of the component target structures and to process pixelvalues within the region of interest to obtain a measurement of theproperty of the component structure,

wherein the processor is arranged to identify the regions of interestsuch that their boundaries fall within image regions corresponding toseparation zones between the component structures within the compositetarget structure.

10. An apparatus of clause 9 wherein the component structures compriseoverlaid gratings formed in two layers on the substrate, and whereindifferent component structures within the composite target are formedwith different overlay bias values.

11. An apparatus of clause 9 or 10 wherein the optical system isarranged to form and detect two or more images of the same compositetarget structure using different portions of the diffracted radiation,and wherein the processor is arranged to compare pixel values fromcorresponding regions of interest identified in the two images to obtaina measurement of asymmetry of the one or more component structures.12. An apparatus of clause 9, 10 or 11 wherein the processor is arrangedto identify regions of interest corresponding to at least two componentstructures in the same detected image and to process their pixel valuestogether to obtain the measurement in accordance with a known biasscheme of the composite target.13. A substrate for use in a method according to any of clauses 1 to 8,the substrate having at least one composite target structure comprisinga plurality of component structures formed on the substrate by alithographic process, wherein the composite target structure is formedwith separation zones between the component structures, wherein withinthe separation zones the composite target structure is formed so as toappear dark in a dark field image of the component structures.14. A substrate of clause 13 wherein the separation zones occupy morethan 5%, optionally more than 10% or more than 15% of the compositestructure in a given direction.15. A substrate of clause 13 or 14 wherein a plurality of compositetarget structures are formed in different layers corresponding to layersof a manufactured device pattern, each composite target structurecontaining component structures in the form of overlaid gratings withdifferent overlay bias values and different orientations.16. A computer program product comprising machine-readable instructionsfor causing a processor to perform the identifying and processing steps(b) and (c) of a method of any of clauses 1 to 8 above.17. An inspection apparatus for measuring a property of a lithographicprocess using a composite target structure including a plurality ofcomponent structures that have been formed by the lithographic processon a substrate, the apparatus comprising:

a support for the substrate having the composite target structure formedthereon;

an optical system for illuminating the composite target structure underpredetermined illumination conditions and for forming and detecting animage of the composite target structure using a predetermined portion ofradiation diffracted by the component target structures under theillumination conditions;

a processor arranged to identify one or more regions of interest in thedetected image, the or each region of interest corresponding to aspecific one of the component target structures and to process pixelvalues within the region of interest to obtain a measurement of theproperty of the component structure,

wherein the processor is arranged to recognize the location of thecomposite target and identify the regions of interest at least in partby recognizing image regions corresponding to separation zones betweenthe component structures within the composite target structure.

18. A lithographic system comprising:

a lithographic apparatus comprising:

-   -   an illumination optical system arranged to illuminate a pattern;    -   a projection optical system arranged to project an image of the        pattern onto a substrate; and    -   an inspection apparatus according to any of clauses 9 to 12 or        17,

wherein the lithographic apparatus is arranged to use the measurementresults from the inspection apparatus in applying the pattern to furthersubstrates.

19. A method of manufacturing devices wherein a device pattern isapplied to a series of substrates using a lithographic process, themethod including inspecting at least one composite target structureformed as part of or beside the device pattern on at least one of thesubstrates using an inspection method of any of clauses 1 to 8 andcontrolling the lithographic process for later substrates in accordancewith the result of the inspection method.20. A method comprising:

detecting an image of a composite target structure using a predeterminedportion of radiation diffracted by component structures underpredetermined illumination conditions;

identifying one or more regions of interest in the detected image, theor each region of interest corresponding to a specific one of thecomponent structures; and

processing pixel values within the region of interest to obtain ameasurement of the property of the component structure,

wherein the composite target structure is formed with separation zonesbetween the component structures so that a variation of a position ofthe one or more regions of interest does not significantly influence theobtained measurement of said property.

21. The method of clause 20, wherein in the processing the regions ofinterest are selected with their boundaries falling within the imageregions corresponding to the separation zones.

22. The method of clause 20, wherein the component structures compriseoverlaid gratings, and wherein different component structures within thecomposite target are formed with different overlay bias values.

23. The method of clause 22, wherein the component structures compriseoverlaid gratings, and different component structures within thecomposite target are formed with different orientations to measureoverlay in different directions.

24. The method of clause 20, wherein two or more images of the compositetarget structure are detected using different portions of the diffractedradiation, and the method further comprises comparing the pixel valuesfrom corresponding regions of interest identified in the images toobtain a measurement of asymmetry of the one or more componentstructures.25. The method of clause 20, wherein in the identifying and processing,regions of interest corresponding to at least two component structuresare identified in the same detected image and their pixel values areprocessed separately.26. The method of clause 20, wherein the separation zones occupy morethan 5%, more than 10% or more than 15% of the composite structure in agiven direction.27. The method of clause 20, wherein the separation zones in thecomposite target structure contain filling structures having an averagedensity similar to that of the component structures but with higherspatial frequencies, whereby radiation diffracted by the fillingstructures falls outside the portion of radiation used in the formationof the detected image.28. An inspection apparatus comprising:

a support configured to support a substrate having a composite targetstructure formed thereon;

an optical system configured to illuminate the composite targetstructure under predetermined illumination conditions and configured toform and detecting an image of the composite target structure using apredetermined portion of radiation diffracted by component structuresunder the illumination conditions; and

a processor arranged to identify one or more regions of interest in thedetected image, the or each region of interest corresponding to aspecific one of the component structures and to process pixel valueswithin the region of interest to obtain a measurement of the property ofthe component structure,

wherein the processor is arranged to identify the regions of interestsuch that their boundaries fall within image regions corresponding toseparation zones between the component structures within the compositetarget structure.

29. The apparatus of clause 28, wherein:

the component structures comprise overlaid gratings formed in two layerson the substrate, and different component structures within thecomposite target are formed with different overlay bias values.

30. The apparatus of clause 28, wherein:

the optical system is arranged to form and detect two or more images ofthe same composite target structure using different portions of thediffracted radiation, and

the processor is arranged to compare pixel values from correspondingregions of interest identified in the two images to obtain a measurementof asymmetry of the one or more component structures.

31. The apparatus of clause 28, wherein the processor is arranged toidentify regions of interest corresponding to at least two componentstructures in the same detected image and to process their pixel valuestogether to obtain the measurement in accordance with a known biasscheme of the composite target.32. A substrate comprising:

at least one composite target structure comprising a plurality ofcomponent structures formed on the substrate by a lithographic process,

wherein the composite target structure is formed with separation zonesbetween the component structures,

wherein within the separation zones the composite target structure isformed so as to appear dark in a dark field image of the componentstructures.

33. The substrate of clause 32, wherein the separation zones occupy morethan 5%, more than 10% or more than 15% of the composite structure in agiven direction.

34. The substrate of clause 32, wherein a plurality of composite targetstructures are formed in different layers corresponding to layers of amanufactured device pattern, each composite target structure containingcomponent structures in the form of overlaid gratings with differentoverlay bias values and different orientations.35. A computer readable medium having stored thereon computer-executableinstructions, execution of which by a computing device causes thecomputing device to perform operations comprising:detecting an image of a composite target structure using a predeterminedportion of radiation diffracted by component structures underpredetermined illumination conditions;

identifying one or more regions of interest in the detected image, theor each region of interest corresponding to a specific one of thecomponent structures; and

processing pixel values within the region of interest to obtain ameasurement of the property of the component structure,

wherein the composite target structure is formed with separation zonesbetween the component structures,

wherein within the separation zones the composite target structure isformed so as to provide regions in the image detected that are notsignificantly influenced by radiation diffracted by the componentstructures.

36. An inspection apparatus comprising:

a support configured to support a substrate having a composite targetstructure formed thereon;

an optical system configured to illuminate the composite targetstructure under predetermined illumination conditions and to form anddetect an image of the composite target structure using a predeterminedportion of radiation diffracted by component structures under theillumination conditions; and

a processor arranged to identify one or more regions of interest in thedetected image, the or each region of interest corresponding to aspecific one of the component structures and to process pixel valueswithin the region of interest to obtain a measurement of the property ofthe component structure,

wherein the processor is arranged to recognize the location of thecomposite target and identify the regions of interest at least in partby recognizing image regions corresponding to separation zones betweenthe component structures within the composite target structure.

37. A lithographic system comprising:

a lithographic apparatus comprising:

-   -   an illumination optical system arranged to illuminate a pattern;    -   a projection optical system arranged to project an image of the        pattern onto a substrate; and    -   an inspection apparatus comprising,        -   a support configured to support a substrate having a            composite target structure formed thereon;        -   an optical system configured to illuminate the composite            target structure under predetermined illumination conditions            and configured to form and detecting an image of the            composite target structure using a predetermined portion of            radiation diffracted by component structures under the            illumination conditions; and        -   a processor arranged to identify one or more regions of            interest in the detected image, the or each region of            interest corresponding to a specific one of the component            structures and to process pixel values within the region of            interest to obtain a measurement of the property of the            component structure,        -   wherein the processor is arranged to identify the regions of            interest such that their boundaries fall within image            regions corresponding to separation zones between the            component structures within the composite target structure;    -   wherein the lithographic apparatus is arranged to use the        measurement results from the inspection apparatus in applying        the pattern to further substrates.        38. A method of manufacturing devices comprising:        applying a pattern to a series of substrates using a        lithographic process;        inspecting at least one composite target structure formed as        part of or beside the device pattern on at least one of the        substrates using an inspection method comprising,        detecting an image of a composite target structure using a        predetermined portion of radiation diffracted by component        structures under predetermined illumination conditions;

identifying one or more regions of interest in the detected image, theor each region of interest corresponding to a specific one of thecomponent structures; and

processing pixel values within the region of interest to obtain ameasurement of the property of the component structure,

wherein the composite target structure is formed with separation zonesbetween the component structures,

wherein within the separation zones the composite target structure isformed so as to provide regions in the image detected that are notsignificantly influenced by radiation diffracted by the componentstructures; and

controlling the lithographic process for later substrates in accordancewith the result of the inspection method.

39. A method of measuring a property of a lithographic process, using acomposite target structure including a plurality of component structuresthat have been formed by the lithographic process on a substrate, themethod comprising the steps of:

(a) forming and detecting an image of the composite target structureusing a predetermined portion of radiation diffracted by the componenttarget structures under predetermined illumination conditions;

(b) identifying one or more regions of interest in the detected image,the or each region of interest corresponding to a specific one of thecomponent target structures; and

(c) processing pixel values within the region of interest to obtain ameasurement of the property of the component structure,

wherein the composite target structure is formed with separation zonesbetween the component structures, wherein within the separation zonesthe composite target structure is formed so as to provide regions in theimage detected in step (b) that are not significantly influenced byradiation diffracted by the component structures.

40. A method comprising:

detecting an image of a composite target structure using a predeterminedportion of radiation diffracted by component structures underpredetermined illumination conditions;

identifying one or more regions of interest in the detected image, theor each region of interest corresponding to a specific one of thecomponent structures; and

processing pixel values within the region of interest to obtain ameasurement of the property of the component structure,

wherein the composite target structure is formed with separation zonesbetween the component structures,

wherein within the separation zones the composite target structure isformed so as to provide regions in the image detected that are notsignificantly influenced by radiation diffracted by the componentstructures.

Although specific reference may have been made above to the use ofembodiments of the present invention in the context of opticallithography, it will be appreciated that the present invention may beused in other applications, for example imprint lithography, and wherethe context allows, is not limited to optical lithography. In imprintlithography a topography in a patterning device defines the patterncreated on a substrate. The topography of the patterning device may bepressed into a layer of resist supplied to the substrate whereupon theresist is cured by applying electromagnetic radiation, heat, pressure ora combination thereof. The patterning device is moved out of the resistleaving a pattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.,having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g., having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the present invention that others can, byapplying knowledge within the skill of the art, readily modify and/oradapt for various applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description by example, and not oflimitation, such that the terminology or phraseology of the presentspecification is to be interpreted by the skilled artisan in light ofthe teachings and guidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

It is to be appreciated that the Detailed Description section, and notthe Summary and Abstract sections, is intended to be used to interpretthe claims. The Summary and Abstract sections may set forth one or morebut not all exemplary embodiments of the present invention ascontemplated by the inventor(s), and thus, are not intended to limit thepresent invention and the appended claims in any way.

The present invention has been described above with the aid offunctional building blocks illustrating the implementation of specifiedfunctions and relationships thereof. The boundaries of these functionalbuilding blocks have been arbitrarily defined herein for the convenienceof the description. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description and not of limitation, suchthat the terminology or phraseology of the present specification is tobe interpreted by the skilled artisan in light of the teachings andguidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

The invention claimed is:
 1. An inspection apparatus for measuring aproperty of a lithographic process using a composite target structureincluding a plurality of component structures that have been formed bythe lithographic process on a substrate, the apparatus comprising: asupport for the substrate having the composite target structure formedthereon; an optical system configured to illuminate the composite targetstructure under predetermined illumination conditions and for formingand detecting an image of the composite target structure using apredetermined portion of radiation diffracted by the plurality ofcomponent structures under the illumination conditions; a processorconfigured to: identify one or more regions in the detected image, eachregion corresponding to a respective one of the plurality of componentstructures, process pixel values within the one or more regions toobtain a measurement of the property of the lithographic process, andidentify the one or more regions such that their boundaries fall withinimage regions corresponding to separation zones between the plurality ofcomponent structures within the composite target structure.
 2. Theapparatus of claim 1, wherein the plurality of component structurescomprise overlaid gratings formed in two layers on the substrate, andwherein different component structures within the composite targetstructure are formed with different overlay bias values.
 3. Theapparatus of claim 1, wherein the optical system is configured to formand detect two or more images of the composite target structure usingdifferent portions of the diffracted radiation, and wherein theprocessor is further configured to compare the pixel values fromcorresponding regions identified in the two or more images to obtain ameasurement of asymmetry of the plurality of component structures. 4.The apparatus of claim 1, wherein the processor is further configured toidentify regions corresponding to at least two component structures inthe detected image and to process their pixel values together to obtainthe measurement in accordance with a known bias scheme of the compositetarget structure.
 5. A lithographic system, comprising: a lithographicapparatus comprising: an illumination optical system arranged toilluminate a pattern; a projection optical system arranged to project animage of the pattern onto a substrate; and an inspection apparatuscomprising: a support for the substrate having a composite targetstructure including a plurality of component structures formed thereon,an optical system configured to illuminate the composite targetstructure under predetermined illumination conditions and for formingand detecting an image of the composite target structure using apredetermined portion of radiation diffracted by the plurality ofcomponent structures under the illumination conditions, and a processorconfigured to: identify one or more regions in the detected image, eachregion corresponding to a respective one of the plurality of componentstructures, process pixel values within the one or more regions toobtain a measurement of a property of the plurality of componentstructures, and identify the one or more regions such that theirboundaries fall within image regions corresponding to separation zonesbetween the plurality of component structures within the compositetarget structure, wherein the lithographic apparatus is configured touse the measurement results from the inspection apparatus in applyingthe pattern to further substrates.
 6. The lithographic system of claim5, wherein: the plurality of component structures comprise overlaidgratings formed in two layers on the substrate, and different componentstructures within the composite target structure are formed withdifferent overlay bias values.
 7. The inspection apparatus of claim 5,wherein the plurality of component structures comprise overlaid gratingsand different component structures within the composite target structureare formed with different orientations to measure overlay in differentdirections.
 8. The lithographic system of claim 5, wherein: the opticalsystem is configured to form and detect two or more images of thecomposite target structure using different portions of the diffractedradiation, and the processor is further configured to compare the pixelvalues from corresponding regions identified in the two or more imagesto obtain a measurement of asymmetry of the plurality of componentstructures.
 9. The lithographic system of claim 5, wherein the processoris further configured to identify regions corresponding to at least twocomponent structures in the detected image and to process their pixelvalues together to obtain the measurement in accordance with a knownbias scheme of the composite target structure.
 10. The apparatus ofclaim 1, wherein the plurality of component structures comprise overlaidgratings and different component structures within the composite targetstructure are formed with different orientations to measure overlay indifferent directions.
 11. The apparatus of claim 1, wherein theseparation zones occupy more than 5% of the composite target structurein a given direction.
 12. The apparatus of claim 1, wherein theseparation zones occupy more than 10% of the composite target structurein a given direction.
 13. The apparatus of claim 1, wherein theseparation zones occupy more than 15% of the composite target structurein a given direction.
 14. The apparatus of claim 1, wherein theseparation zones in the composite target structure contain fillingstructures having an average density similar to that of the plurality ofcomponent structures but with higher spatial frequencies, wherebyradiation diffracted by the filling structures falls outside the portionof radiation used in the formation of the detected image.
 15. Aninspection apparatus for measuring a property of a lithographic processusing a composite target structure including a plurality of componentstructures that have been formed by the lithographic process on asubstrate, the apparatus comprising: a support for the substrate havingthe composite target structure formed thereon; an optical systemconfigured to illuminate the composite target structure underpredetermined illumination conditions and for forming and detecting animage of the composite target structure using a predetermined portion ofradiation diffracted by the plurality of component structures under theillumination conditions; a processor configured to: identify one or moreregions in the detected image, each region corresponding to a respectiveone of the plurality of component structures, process pixel valueswithin the one or more regions to obtain a measurement of the propertyof the lithographic process, and recognize the location of the compositetarget and identify the one or more regions at least in part byrecognizing image regions corresponding to separation zones between theplurality of component structures within the composite target structure.16. The inspection apparatus of claim 15, wherein the plurality ofcomponent structures comprise overlaid gratings formed in two layers onthe substrate, and wherein different component structures within thecomposite target structure are formed with different overlay biasvalues.
 17. The inspection apparatus of claim 15, wherein the pluralityof component structures comprise overlaid gratings and differentcomponent structures within the composite target structure are formedwith different orientations to measure overlay in different directions.18. The inspection apparatus of claim 15, wherein the separation zonesoccupy more than 5% of the composite target structure in a givendirection.
 19. The inspection apparatus of claim 15, wherein theseparation zones in the composite target structure contain fillingstructures having an average density similar to that of the plurality ofcomponent structures but with higher spatial frequencies, wherebyradiation diffracted by the filling structures falls outside the portionof radiation used in the formation of the detected image.